The field of the invention is electrical machines including motors and generators and the application of superconducting technologies to such machines.
The need for a high power, small volume, high efficiency, low cost, reliable electric machine is well understood. The applications include, but are not limited to, ship propulsion, industrial drives, power generation and transportation. The development of high temperature superconducting (HTS) wires offers a bright future for superconducting devices. In this context, xe2x80x9chigh temperaturexe2x80x9d means operation above a temperature of about 40 degrees Kelvin, which is still well below zero degrees Fahrenheit. A well known superconducting cooling assembly is referred to as a cryostat.
Dombrovski et al., U.S. Pat. No. 6,313,556, issued Nov. 6, 2001, discloses an AC synchronous motor with a stator, a rotor and a rotor winding of HTS wires which are cooled by a cryostat. The cryostat includes a refrigeration system that is connected through conduits in an extension of the rotor shaft into a vacuum chamber in the interior of the rotor. A cryogenic transfer coupling is provided to allow that portion of the superconducting structure which is inside the rotor to rotate with the rotor.
The reliability of a rotating cryostat is lower than that of a stationary cryostat for a comparable cost of manufacture. In the rotating system, the superconducting coils not only have to bear the motor torque but also have to be thermally isolated. This places a stringent requirement on the mechanical design which translates into a complicated design for the cryostat, with lower reliability and increased cost. Furthermore, when the machines of the prior art experience a failure in the superconducting system and lose the excitation flux, the motor cannot be rotated. The so-called xe2x80x9chop-along capability,xe2x80x9d which is the ability to rotate the rotor of a motor even under failure conditions, is practically zero.
Recently, a new type of machine referred to as a high strength undiffused brushless machine has been disclosed in Hsu U.S. application Ser. No. 09/872,048, filed Jun. 1, 2001. In this machine, additional excitation is provided by a stationary excitation winding which is positioned next to the rotor so as to induce a rotor-side flux in the rotor. Permanent magnet (PM) material is positioned in between the poles of the rotor to control the flux diffusion of the secondary flux produced by the additional excitation. The flux provided by the stationary excitation winding is thus available to increase or decrease a resultant flux in the main air gap. This invention is applicable to both axial gap and radial gap machines.
In known superconducting machines there is also a problem that back iron requirements are large in order to isolate the alternating flux. This adds to the weight of a high temperature superconducting motor. The overall wound core length that includes the core length and the length of the two winding end turns of a known superconducting machine may be quite long. Even when helical coils are used instead of the conventional armature coils, the overall core length is still quite long for a machine with lower numbers of poles in comparison with a PM machine that is capable for a machine with higher number of poles.
There are also limitations to increasing the number of poles in the prior art machines. Unlike a PM motor, the number of poles of a non-PM superconducting motors cannot be increased to a number as high as that of a PM motor. This is because the armature flux of a PM motor is guided by the iron but the armature flux of a non-PM superconducting motor flows freely in the air.
The present invention has been made to overcome the limitations of the prior art.
Prior superconducting motors and generators have used a rotating cryostat. The present invention uses a stationary cryogenic assembly together with permanent magnets (PM) to provide a new type of superconducting motors and generators. The problems associated with the conventional superconducting machines and conventional PM machines, such as reliability, hop-along capability, mechanical stress, cost, thermal isolation, and manufacturability problems, are significantly overcome through this new technology.
In the machine of the present invention, torque does not act on the superconducting coils. The present invention can use either high temperature or low temperature superconducting wires.
Unlike the prior superconducting machines, the superconducting machine of the present invention has a hop-along capability as a result of the presence of the PM material.
Unlike the prior superconducting machines, the superconducting machine of the present invention can be built with a high number of poles and with less length of superconducting wire than prior high temperature superconducting machines.
The machine of the present invention can be an axial-gap machine, a radial-gap machine or a radial-gap inverted machine. The inverted machine has an armature disposed in a central chamber in the rotor, which is separated from the armature by the main air gap. A stationary cryogenic assembly encircles the rotor and is spaced from the rotor by a secondary air gap.
The machine of the present invention can be built as a motor or a generator.
The machine of the present invention can use a multiple DC excitation coil arrangement.
The superconducting machine of the present invention can be built as either a disc-shaped or a barrel-shaped machine.
Unlike a conventional PM machine, the main air gap flux of this new machine can be weakened through control of the secondary excitation.
Unlike a conventional PM machine, the main air gap flux density of this new machine can be enhanced through control of the secondary excitation.
Unlike a conventional PM machine, the manufacturability of this new machine can be ensured by a predetermined excitation or by a magnetic short-circuiting approach.
The flux produced by the superconducting coils in the machine of the present invention is returned through the air, without requirements for back iron.
These and other objects and advantages of the invention will be apparent from the description that follows and from the drawings which illustrate embodiments of the invention, and which are incorporated herein by reference.